1. Field of the Invention
The invention relates to a method of processing a digital image in order to construct, by means of a geometrical perspective transformation applied to an image termed the source image, a calculated image, termed the target image, which represents the source image reconstructed with modifications of the tilting angle and of the panning angle, and possibly of the scale factor in relation to the source image.
The invention also relates to a device for processing digital images which implements this method.
The invention finds its application in the construction of surveillance systems comprising a certain number of fixed cameras arranged in such a way that their individual fields of view blend to form a wide-angle field of view, for observing a panoramic scene.
2. Description of the Related Art
A process, capable of culminating in a device, for carrying out a geometrical perspective transformation is already known from the publication "Computer Image Processing and Recognition" by Ernest L. HALL, in "Academic Press 1979", "A Subsidiary of Harcourt Brace Jonanovitch Publishers, New York, London, Toronto, Sydney, San Francisco"--Chapter 3, pp. 76 et seq., especially pp.76-88.
This publication teaches the definitions pertaining to perspective geometry and to a perspective transformation and an inverse perspective transformation, depending on whether it is imagined that we are passing from the points of an image to the light rays passing through these points, or else that we are passing from the light rays to the points of the image. The operation of passing from the points of an image to the rays which correspond to these points is called "perspective transformation".
This publication teaches a process firstly for formulating the problem of passing from points to rays, and next for solving this "perspective transformation" problem using 4.times.4 matrices. This problem is solved with a complete analytical calculation based on these matrices.
The formalizing of the problem by means of 4.times.4 matrices is based on the following considerations: theoretically, perspective transformations are geometrical problems which are posed in 3-dimensional space, hence using 3 types of coordinates only; in this publication, it is taught that the perspective transformation is not a linear transformation, and this results in difficulties in handling the equations; this publication then teaches that the solving of the equations can, however, be simplified, by linearizing these equations, this being possible if a fourth variable is added; thus, these linearized equations can be solved analytically.
Nevertheless, the process taught by the cited publication leads to solutions with extremely complex formulations. Examples are given throughout the cited publication.
Now, according to the present invention, a first constraint is imposed involving the implementation of perspective transformations in real time, that is to say while processing an image at the rate of 16 to 25 images formed per second.
It appears that the solutions formulated according to the teaching of the cited publication are particularly difficult to implement in real time, owing to their complexity.
There are however electronic devices called "transform chips" which are available commercially and capable of performing geometrical transformations and which can be used to solve the problem of perspective transformations.
In particular, a commercial product is known, referenced TMC 2302, available from TRW Company whose address is PO Box 2472, LA JOLLA, Calif. 92 038 (US). This product is capable of performing very complicated calculations in real time, relating to the geometrical transformations required for the envisaged perspective transformations.
However, these transform chips are expensive, especially because of the tact that they are not widely used; there is little hope of seeing their cost come down soon. Moreover, their implementation within a signal processing device such as that envisaged according to the invention must be regarded as involving a number of constraints which are difficult to overcome.
Nevertheless, having overcome these constraints, reconstruction of the target image, as defined in the preamble, gives a result which might be termed reconstruction according to an "exact perspective". By using the transform chips, the objects in the target image are seen in a geometrical perspective which is very close to that which would be obtained if this target image were formed directly, that is to say by means of a real camera oriented together with the parameters producing the geometrical perspective chosen for this image termed target image.